Bhattacharjee M K, Mayer R M
Department of Chemistry, Ohio State University, Columbus 43210.
Carbohydr Res. 1993 Apr 7;242:191-201. doi: 10.1016/0008-6215(93)80034-c.
Dextransucrase from Streptococcus sanguis 10558 was found to synthesize alpha-(1-->6), alpha-(1-->3), and alpha-(1-->2) linkages during an acceptor-dependent glucosyl transfer reaction. Normally, new glucosyl residues are added at C-6 of monosaccharide acceptors. However, sugars blocked at C-6 also can serve as good acceptors. The disaccharide and trisaccharide products formed when methyl 6-bromo-6-deoxy-alpha-D-glucopyranoside was used as acceptor were isolated and characterized. Both were found to contain only alpha-(1-->3) glycosidic bonds. This supports the hypothesis that when C-6 is blocked the acceptor binds to the enzyme in a flipped orientation, resulting in an approximate exchange in space of the C-3 and C-6, thereby putting C-3 adjacent to the active site. The second alpha-(1-->3) links in the trisaccharide are formed by a single-chain mechanism without release of the intermediate disaccharide. With maltose as acceptor, new glucosyl residues are added at C-6'. However, if that position is blocked with a bromine atom, the resulting compound, 6'-bromo-6'-deoxy-maltose, can still serve as an acceptor. The product in this case was isolated and characterized. The new glycosidic link was found to be alpha-(1-->2).
研究发现，血链球菌10558来源的葡聚糖蔗糖酶在依赖受体的糖基转移反应中能合成α-(1→6)、α-(1→3)和α-(1→2)糖苷键。通常情况下，新的葡萄糖基残基会添加到单糖受体的C-6位。然而，C-6位被封闭的糖类也可作为良好的受体。以6-溴-6-脱氧-α-D-吡喃葡萄糖苷甲酯作为受体时形成的二糖和三糖产物被分离并进行了表征。结果发现二者均只含有α-(1→3)糖苷键。这支持了以下假说：当C-6位被封闭时，受体以翻转的方向与酶结合，导致C-3和C-6在空间上近似交换，从而使C-3靠近活性位点。三糖中的第二个α-(1→3)键是通过单链机制形成的，中间二糖不会释放。以麦芽糖作为受体时，新的葡萄糖基残基会添加到C-6'位。然而，如果该位置被溴原子封闭，生成的化合物6'-溴-6'-脱氧麦芽糖仍可作为受体。对这种情况下的产物进行了分离和表征。发现新的糖苷键为α-(1→2)。
